a. Field of the Invention
This invention relates to a braking system for a vehicle. In particular, the invention relates to a braking system that enables application and release of a parking brake with fluid control and that also maintains application of the brake during a loss of fluid pressure—all without using spring applied braking and using only a single fluid control conduit.
b. Background Art
Conventional commercial vehicle braking systems typically include spring-actuated/fluid released parking brake actuators in which a mechanical spring actuates or applies the parking brake and pressurized fluid (typically pneumatic fluid) is used to release the brake. The parking brake remains released as long as fluid pressure is maintained in the parking brake actuator. The braking system typically has a common source of fluid pressure connected to actuators disposed at each wheel end of the of vehicle such that fluid pressure in a common control line simultaneously releases the parking brake for each wheel.
The use of spring-actuated/fluid released parking brake actuators has several drawbacks. An unintended loss of fluid pressure due to, for example, a leak or rupture in the fluid control system results in actuation of the parking brakes on each wheel. While this action is desirable—and even required in certain circumstances—to meet safety concerns, the action is undesirable in many other circumstances. For example, in military applications it is not uncommon for the fluid control system to become damaged as a result of damage to a single wheel from an improvised explosive device (IED). Actuation of the parking brakes in this circumstance immobilizes the vehicle and may trap the vehicle occupants in a hostile environment. In other applications, movement of the vehicle may be required because the immobilized vehicle is in a dangerous location (e.g., with high traffic volumes), is blocking the passage of other vehicles including potential emergency service vehicles, or is in a remote location far from potential repair facilities (e.g., a logging vehicle in a remote forest). The spring in the actuator is relatively heavy, highly stressed to produce the parking force and also tends to corrode and fatigue over time. The corrosion makes repair and replacement different. More importantly, because of the significant spring forces applied in the actuator, unexpected failure of the spring may result in significant injury to maintenance and other personnel. Finally, constant fluid pressure is required to maintain the parking brake in a release position.
In order to overcome some of the problems associated with spring-actuated/fluid released parking brake actuators, some actuators have been developed that use fluid pressure to actuate the parking brake. One example is the brake actuator previously offered for sale under the trademark DD-3 by the applicant Bendix Commercial Vehicle Systems LLC. For proper functioning of the parking brake, fluid actuated parking brake actuators must include means to maintain application of the parking brake despite any loss in fluid pressure. In the case of the DD-3 and similar brake actuators, a separate fluid control line has been used to actuate the locking mechanism. The requirement for an additional fluid control line, however, increases the number of parts, cost, and complexity of the braking system.
The inventors herein have recognized a need for a braking system that will minimize and/or eliminate one or more of the above-identified deficiencies.